A conventional magnetic shielding apparatus for shielding an ambient magnetic field (external magnetic field), used in measurement of biomagnetism generated from a living body, has been built as a spatially closed chamber in a box-like shape, formed by tightening a sheet material high in permeability (a high-permeability material such as Permalloy, and so forth) up to a frame made of aluminum, and stainless steel, without any gap interjacent there between, by use of bolts or the like so as to be fixedly attached thereto. Further, a sheet material of Permalloy has been disposed in multiple layers to enhance a magnetic shielding factor, and a sheet material large in electric conductivity (aluminum, and so forth) has been used to shield electromagnetic waves. There has been reported a cylindrical magnetic shielding apparatus small in size and light in weight, using a high-permeability sheet material instead of Permalloy (refer to, for example, JP-A No. 136492/2002).
A high-permeability material generally refers to material having a relative magnetic permeability in a range of about 10,000 to 100,000. More specifically, Permalloy, silicon steel, and so forth are heavily used, and these materials are also called as a μ-metal. Those high-permeability materials have a property of allowing magnetic fluxes to easily penetrate there through, and this property is applied to magnetic shielding. For example, as shown in FIG. 18, when a cylinder 182 made of a high-permeability material is placed in an ambient magnetic field uniform in the vertical direction, lines of magnetic force 181 have distribution at a higher density within the high-permeability material than that in the air. As a result, a space loose in magnetic flux density occurs on the inner side of the cylinder 182, that is, there is formed a space that has shielded an external magnetic field. This is a fundamental principle behind magnetic shielding with the use of the high-permeability material.
Meanwhile, when an AC magnetic field is applied to an electrically conductive material such as metal, alloy, and so forth, a shielding current, so-called eddy current, is known to spontaneously occur within the material. The higher electric conductivity of the material, the greater an effect of the eddy current is, so that copper and aluminum are generally used as a magnetic shield. Further, it is known that the greater an area of a plane orthogonal to the direction of propagation of an electromagnetic wave, the greater the effect of the eddy current is.
Further, it is known that a magnetic shielding factor of the cylindrical magnetic shielding apparatus is higher in a direction orthogonal to a cylindrical axis than that in the direction of the cylindrical axis. Accordingly, in the case of carrying out measurement of a magnetic field inside the cylindrical magnetic shielding apparatus, a magnetic field component in the direction orthogonal to the cylindrical axis is measured more often than not. In the case of biomagnetism measurement for detecting a minute magnetism, use is generally made of a SQUID flux meter using a superconducting quantum interference device (SQUID). Further, there have been proposed two methods (refer to, for example, JP-A No. 136492/2002) for inserting an inspection target inside the cylindrical magnetic shielding apparatus, that is, (1) a method whereby the inspection target is inserted through open ends at respective ends of the cylindrical magnetic shielding apparatus, and (2) a method whereby the cylindrical magnetic shielding apparatus is provided with a door that can be opened and closed, and the inspection target is inserted through the door.
In general, the conventional cylindrical magnetic shielding apparatus in the form of a box-like chamber has a floor about 2 m in longitudinal as well as transverse dimension, and is about 2 m in height, weighing heavily, so that there has been a problem in that there are limitations to a room where the apparatus can be installed, and in addition, a cost thereof is high.
The method for inserting the inspection target inside the cylindrical magnetic shielding apparatus under (1) as above has an advantage in that since no door is installed, a structure becomes simpler, however, the method has had a problem in that an inspection engineer finds it difficult to observe the interior condition of the magnetic shielding apparatus, and to accurately adjust an inspection position. Furthermore, as there is a need for pulling out a bed in order to place the inspection target thereon, there is a need for preparing a room about 4 m in length in order to enable the magnetic shielding apparatus to be installed therein on the assumption that the magnetic shielding apparatus, in the direction of the cylindrical axis, is about 2 m in length.